I doubt that it makes sense to invest that kind of money in an 22 year old 27 foot boat, especially one like the Bucaneer, but if you have your heart set on installing a small inboard engine, you would probably do best with a saildrive type rather than a conventional shaft drive. They are easier to install and do not have the alignment problems likely to plague a boat that was not engineered for an inboard. I have a 10 hp Buhk in my boat. It is a nice little engine that has been quite reliable. Volvo and Yanmar also used to make small single cyl. diesel saildrive type units but I have no real experience with them.
That said, installing an inboard diesel in an older boat is a major operation both from a cost and labor standpoint. Even if you could find a nice small diesel somewhere it would cost more than a Bucaneer 275 is worth. I think the easiest and less expensive way to get a boat with an inboard is to sell the Bucaneer and buy a boat with an inboard already installed.